X11: Assume 96 DPI if RandR monitor size is zero

This falls back to calculating the monitor physical size from the
current resolution and the default X11 DPI when the physical size
returned by RandR is zero.

src/x11_monitor.c

@@ -161,6 +161,16 @@ void _glfwPollMonitorsX11(void)
                 heightMM = oi->mm_height;
             }
 
+            if (widthMM <= 0 || heightMM <= 0)
+            {
+                // HACK: If RandR does not provide a physical size, assume the
+                //       X11 default 96 DPI and calcuate from the CRTC viewport
+                // NOTE: These members are affected by rotation, unlike the mode
+                //       info and output info members
+                widthMM  = (int) (ci->width * 25.4f / 96.f);
+                heightMM = (int) (ci->height * 25.4f / 96.f);
+            }
+
             _GLFWmonitor* monitor = _glfwAllocMonitor(oi->name, widthMM, heightMM);
             monitor->x11.output = sr->outputs[i];
             monitor->x11.crtc   = oi->crtc;